1. In a recent paper on “The Effective Inertia of Electrified Systems moving with High Speed,” G. W. Walker extends his previous investigation to the case of a perfectly conducting oblate spheroid, whose axis lies in the direction of motion, for the time being, of its centre, and whose eccentricity is equal to
k
, where
k
denotes the ratio of the speed of the centre to that of light, also for the instant under consideration. He finds (Note 1) that the longitudinal and transverse masses are equal respectively to 2
e
2
( 1 —1/5
k
2
)/3
ac
2
(1 —
k
2
)
3/2
and 2
e
2
( 1 —1/60
k
2
)/3
ac
2
(1 —
k
3
)
1/2
(1) instead of 2
e
2
/ 3
ac
2
( 1 -
k
2
)
3/2
and 2
e
2
/ 3
ac
2
( 1 -
k
2
)
1/2
(2) the values given by the mass formulæ of Lorentz and required by the Principle of Relativity. The method used by Walker obviates the objections raised to the assumption of quasi-stationary motion, which is necessary in the ordinary energy method of calculating the masses, but it requires special assumptions to be made at the outset both as to the constitution of the electron and as to the boundary conditions at its surface whenever definite information is needed. These assumptions obviously limit the generality of the investigation, so that it is open to the objection that Walker's model of the electron is not a suitable one, precisely because it does not agree with the Principle of Relativity.
The electromagnetic inertia of the lorentz electron